On sailboats, it is known to use battens or stiffeners in the sail to increase the amount of sail area for a given mast height and boom length. This extra sail area maintained by battens is called roach. Battens are especially effective for adding sail area up high on a sail. This sail area is very effective at keeping the top of the sail flat and forcing the top of the sail to twist when it is hit by a wind gust. This twisting action reduces the angle of attack of the sail. The combination of less angle of attack and flatter sail reduces the lift and drag generated by the top of the sail which reduces the tendency to heel the boat over. The extra sail area also makes the planform of the sail more closely approximate an ellipse. A sail with elliptical tip is much more efficient than a sail of triangular shape. This relatively recent innovation in sail shape is called a square top sail.
Sailboats also use battens to improve the sail shape. Battens can prevent the sail from luffing and the batten stiffness can be varied to affect sail shape.
Battens also reduce wear and tear on the sail and increase the sail life by reducing the amount of flapping in the wind.
Battens make a boomless option much less of a compromise to sailing performance. The boomless option is desirable because it is safer—no chance of getting hit in the head, less expensive and lighter, and operation of the sail boat is made simpler.
Some sailboats have a mast that can rotate and have the sail furl around the mast. This is a valuable feature because a rotatable mast:
1) Improves safety—the sail can be put away quickly if the wind comes up suddenly.
2) Convenience—the sail can be furled quickly for storage.
3) Adjustability—the sail can be partially furled or reefed.
It is impossible to furl the sail around a mast with conventional battens since the battens are too stiff to wrap around the mast. There are several designs that have been tried to get more roach on sails that are intended to be furled or create battens that can furl:
Short battens known as leech battens have been used on sails when positioned at a steep angle. These sails can furl around the mast, but the battens are too short and do not support much roach when the sail is unfurled.
Henderson U.S. Pat. No. 5,333,569 describes a sail using inflatable battens in the form of hollow, flexible tubes. These sails are believed not to have been tested in the market and they are more complicated, involving pressuring pumps and valved fittings, and are slow and inconvenient to use. There is always the concern that there will be a leak.
Stevenson U.S. Pat. No. 4,838,192 describes pivoting battens that can be rotated from near horizontal when the sail is unfurled to vertical when the sail is furled into a bag at the forward edge of the sail by a control line. The battens are each supported by a plurality of guide loops stitched to the sail which allow the battens to move lengthwise with respect to the guide loops. This battened sail is more complicated to manufacture and use.
Skinner U.S. Pat. No. 4,633,798 describes a sail having a plurality of short battens positioned near the leech edge made of a thin band of steel with a cross section that is curved to give the band stiffness much like an ordinary steel tape measure. If the batten is bent beyond a certain point, the cross section is forced to flatten out and then the batten is flexible and can furl around a mast. This approach has not been tested in the market and it is not known if the battens can flex far enough to support a large roach without yielding by bending over or kinking. The sail with steel bands must also be handled with care since repeated furling and unfurling could result in the development of unwanted permanent deformation or creasing of the steel battens.
A square top sail is offered on the Hobie Outback kayak which has a single, steeply angled batten which can furl around the mast but the boat has no means for furling the sail.